This invention relates to an AM stereo receiver.
An AM stereo receiver as described in Japanese Patent Laid-Open No. 140901/1978 has conventionally been known. This AM stereo system is also referred to as an "AM-PM system" in which an amplitude modulation (AM) sum signal L+R and a phase modulation (PM) difference signal L-R are transmitted from a transmitter. The letter L stands for the left channel stereo signal and the letter R designates the right channel stereo signal.
Accordingly, a heretofore-known AM stereo receiver includes an antenna circuit, an RF amplifier stage and an intermediate frequency amplifier stage in the same way as a conventional AM radio receiver, and forms the sum signal (monaural signal) L+R by AM detecting a received signal obtained through the intermediate frequency amplifier stage. On the other hand, the difference signal L-R is formed by PM detecting the received signal. Both signals are composed (added or subtracted) in a matrix circuit to obtain a left channel stereo signal L and a right channel stereo signal R.
In the abovementioned PM detection, an AM modulation component is removed by a limiter amplifier. Consequently, the difference signal L-R formed by the PM detection has a constant level which is unaffected by the RF input field intensity RF.sub.IN, as shown in the characteristic diagram of FIG. 1, whereas the sum signal L+R formed by the AM detection changes in proportion to the RF input field intensity RF.sub.IN as shown in the drawing. If both signals are directly composed by the matrix circuit, satisfactory stereo separation cannot be obtained. Hence, the difference signal L-R formed by the PM detection must be passed through a variable gain circuit which is subject to control by an AGC voltage varying in accordance with the level of the sum signal L+R, in order to realize level matching between both signals.
The inventor of the present invention has determined that the following problem arises in this case.
Namely, as shown in the characteristic diagram of FIG. 2, the above-mentioned AGC voltage V.sub.AGC contains an offset voltage V.sub.offset due to a d.c. bias voltage in a detection element, such as a transistor for the AM detecting operation. Moreover, the offset voltage V.sub.offset in the AGC voltage frequently corresponds to a base bias voltage of the transistor which is level-shifted by the base-to-emitter voltage of the transistor. In other words, the AGC voltage has temperature dependency in accordance with the temperature characteristic of the base-to-emitter voltage of the transistor. This results in the problem that level matching becomes difficult between the difference signal L-R and the sum signal L+R due to the temperature change, and deterioration of stereo separation occurs.